Viral strain HIV-2UC9429 has been shown to be infectious and pathogenic in baboons (Papio species), producing an AIDS syndrome with classical depletion of CD4+ lymphocytes. This virus will be useful for vaccine challenge and therapeutic evaluations in the baboon, as the immunologic physiology of the baboon is very close to the human. Before it can be used in this way, the virus must be titrated in vivo to determine the minimal infectious dose in this species. Six baboons were inoculated intravenously in pairs at doses of 10, 1 and 0.1 TCID. All baboons became infected, even at the lowest dosage. Plans are underway to expand this study to test lower dilutions of virus to determine the minimal infectious dose, before vaccine studies are begun. FUNDING NIH grant RR00166 and NIAID SVEU contract.